The scanning Fabry-Perot etalon spectrometer is a ubiquitous tool for high-resolution optical spectroscopy. In the most common configuration, the optical path length between the etalon mirrors is (thermally or mechanically) scanned through resonant optical wavelengths that are transmitted in time sequential order. Another common option is to tune the angle of incidence at which the light impinges on the etalon to change the passband. Very high-resolution wavelength measurements can be achieved using etalons. Etalons can be used as tunable filters where the tuning is accomplished by either angle or optical path length changes. By continuously tuning these parameters, one can use an etalon as a high-resolution spectrometer.
Although etalons are capable of very high wavelength resolution, in general, they only transmit a very narrow wavelength band at a time coinciding with the current optical path length and angle of incidence (AOI). A key measure of etalon performance is the finesse, the ratio of the free spectral range (FSR) to the full-width at half-maximum (FWHM) transmission bandwidth. A narrower filter (with the same free spectral range) rejects more out of band light. This means that as the resolving power (and finesse) are increased, more light is required to make a narrow-band measurement. In present systems, the light that is reflected is lost and only a very narrow band of light is measured. Scanning allows for the pass-band of the light to be changed but only one passband can be measured at any given time. This means that optical power (or integration time) needs to increase as the filter narrows so improving the resolution comes at the price of optical efficiency. Another common usage is to send a non-collimated beam through an etalon, which yields a well-known ring pattern and passes different wavelengths at different angles but is equally lossy. In contrast, a grating spectrometer splits the light of multiple wavelengths into different angles. This yields simultaneous wavelength measurements but with lower resolution (and generally high loss). Another technology that has seen recent success is the Virtually Imaged Phased Array (VIPA). A VIPA can suffer from some of the same problems as gratings because the light is split into multiple orders and there is insertion loss in getting the light into the VIPA. The most desirable spectrometer is one that has simultaneous, continuous wavelength coverage like a grating spectrometer but maintains the resolution and luminosity of an etalon.